Uni-planer bone fixation assembly

ABSTRACT

A bone fixation assembly includes a plurality of bone fixation elements that each include a bone anchor configured to be implanted into underlying bone, such as a vertebra. Each bone anchor is received in an anchor seat, and the anchor seats are joined by a fixation rod so as to operatively couple and fix the position and orientation of the vertebrae relative to each other. The bone anchor is free to rotate relative to the anchor seat, and is also free to pivot in a desired direction relative to the anchor seat.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/090,752, filed Apr. 5, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/143,489, filed Dec. 30, 2013, now U.S. Pat. No.9,326,796, which is a continuation of U.S. patent application Ser. No.12/611,286, filed Nov. 3, 2009, now U.S. Pat. No. 8,628,558, whichclaims the benefit of U.S. Patent Application Ser. No. 61/110,704, filedNov. 3, 2008, the contents of each of which are hereby incorporated byreference as if set forth in their entirety herein. This is related bysubject matter to PCT Patent Application Serial No. PCT/US2008/070670,having an international filing date of Jul. 21, 2008, the contents ofwhich are hereby incorporated by reference as if set forth in theirentirety herein.

BACKGROUND

Conventional bone fixation elements, such as pedicle screws, include abone anchor retained within an anchor seat and captured by a collet.Pedicle screw assemblies include a plurality of pedicle screws joined bya rod that extends through rod slots formed in the pedicle screws.Uni-planar pedicle screws provide one degree of freedom. That is, thebone anchor retained within the anchor seat and, in some systems, thecollet, is free to move with respect to the anchor seat in only oneplane, e.g., the sagittal plane. Motion of the bone anchor is limited tothis sagittal plane in conventional pedicle screws by a pinning orstaking process during manufacture of the assemblies to create a pivotin the sagittal plane. As a result, the height of the bone screw islimited by the orientation of the rod slot. Unfortunately, if the rodslot isn't in line with the trajectory of the rod, the anchor seat mustbe turned, which results in either advancing or withdrawing the screwtoward and away from the bone surface.

It is therefore desirable to provide a bone fixation element that allowsthe screw head to angulate in a desired plane while also allowing thebone anchor to rotate freely with respect to the anchor seat withoutadvancing or withdrawing the screw toward or away from the bone surface.

SUMMARY

In one embodiment, a bone fixation subassembly is configured to receivea fixation rod and a locking cap. The bone fixation subassembly includesan anchor seat and a collet. The anchor seat includes an anchor seatbody extending along a central axis and defining an upper end and alower end. The upper end includes a pair of opposing fixationrod-receiving gaps therebetween that are spaced along a longitudinalaxis, and a bore disposed between the rod-receiving gaps. The colletincludes a collet body disposed in the anchor seat. The collet body isconfigured to attach to a bone anchor that extends along an axis ofrotation. A bone anchor attached to the collet is permitted to rotateabout the axis of rotation relative to the anchor seat, and the boneanchor is further permitted to pivot relative to the anchor seat along adesired plane. The bone anchor is prevented from pivoting in anotherplane that includes the central axis and is angularly offset withrespect to the desired plane.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe preferred embodiments of the application, will be better understoodwhen read in conjunction with the appended drawings. For the purposes ofillustrating the sagittal pedicle screw systems of the presentapplication, there is shown in the drawings preferred embodiments. Itshould be understood, however, that the application is not limited tothe precise arrangements and instrumentalities shown. In the drawings:

FIG. 1A is a perspective view of a bone fixation assembly constructed inaccordance with one embodiment including a plurality of bone fixationelements connected by a bone fixation rod, and illustrated schematicallyas each being affixed to a vertebra;

FIG. 1B is a perspective view of one of the bone fixation elementsillustrated in FIG. 1A constructed in accordance with one embodiment,including an anchor seat, a bone anchor, a collet, and a locking cap;

FIG. 2 is a perspective view of the bone fixation rod illustrated inFIG. 1A;

FIG. 3 is a perspective view of the bone anchor illustrated in FIG. 1B;

FIG. 4A is a perspective view of the anchor seat illustrated in FIG. 1B;

FIG. 4B is an enlarged portion of a guide provided by the anchor seatillustrated in FIG. 4A;

FIG. 5A is an exploded perspective view of the locking cap illustratedin FIG. 1B;

FIG. 5B is a top plan view of the locking cap illustrated in FIG. 5A;

FIG. 5C is a sectional side elevation view of the locking capillustrated in FIG. 5B;

FIG. 6 is a perspective view of the collet illustrated in FIG. 1B;

FIG. 7A is a sectional side elevation view of the bone fixation elementillustrated in FIG. 1B taken along line 7A-7A, with the locking capremoved, to illustrate a bone fixation subassembly;

FIG. 7B is a sectional side elevation view of the bone fixationsubassembly illustrated in FIG. 7A, and taken along line 7B-7B of FIG.1B;

FIG. 7C is a sectional side elevation view similar to FIG. 7A, butshowing the bone fixation element including a fixation rod extendingthrough the anchor seat, and a locking cap affixed to the anchor seat;

FIG. 7D is a sectional side elevation view similar to FIG. 7B, butshowing the bone fixation element illustrated in FIG. 7C;

FIG. 8A is a schematic view illustrating a method for assembling thebone fixation element illustrated in FIG. 1A;

FIG. 8B is a schematic view illustrating the method for assembling thebone fixation element illustrated in FIG. 1A, and showing the colletbeing inserted into the axial bore of the bone anchor;

FIG. 8C is a schematic view illustrating the method for assembling thebone fixation element illustrated in FIG. 1A, and showing an upwardforce being applied to the bone anchor so as to insert the anchor headinto the lower end of the collet;

FIG. 8D is a schematic view illustrating the method for assembling thebone fixation element illustrated in FIG. 1A, and showing a downwardforce being applied to the collet thereby locking the anchor and colletin the anchor seat;

FIG. 9A is a sectional side elevation view of a bone fixationsubassembly constructed in accordance with an alternative embodiment,taken from the same orientation as FIG. 7A;

FIG. 9B is a sectional side elevation view of the bone fixationillustrated in FIG. 9A, taken from the same orientation as FIG. 7B;

FIG. 10A is a top perspective view of a collet of the bone fixationsubassembly illustrated in FIGS. 9A-B;

FIG. 10B is a bottom perspective view of the collet illustrated in FIG.10A;

FIG. 11A is a perspective view of a bone fixation subassembly includingan anchor seat extension constructed in accordance with an alternativeembodiment;

FIG. 11B is a perspective view of the anchor seat extension illustratedin FIG. 11A;

FIG. 11C is a perspective view of the bone fixation subassemblyillustrated in FIG. 11A, with a potion cut away;

FIG. 12A, is a sectional side elevation view of the bone fixationsubassembly illustrated in FIG. 11A, taken along line 12A-12A;

FIG. 12B, is a sectional side elevation view of the bone fixationsubassembly illustrated in FIG. 11A, taken along line 12B-12B;

FIG. 13A is a schematic view illustrating a method for assembling thebone fixation subassembly illustrated in FIG. 11;

FIG. 13B is a schematic view illustrating the method for assembling thebone fixation subassembly illustrated in FIG. 11, and showing the anchorseat extension being attached to the bone anchor by clipping the collarover the anchor neck;

FIG. 13C is a schematic view illustrating the method for assembling thebone fixation subassembly illustrated in FIG. 11, and showing the anchorbody being brought down onto the anchor head;

FIG. 14A is a sectional side elevation view similar to FIG. 12A, but ofa bone fixation subassembly constructed in accordance with anotherembodiment including a collet extension;

FIG. 14B is a sectional side elevation view similar to FIG. 12A, but ofthe bone fixation subassembly illustrated in FIG. 14A;

FIG. 15 is a perspective view of the collet extension illustrated inFIG. 14A;

FIG. 16A is a schematic view illustrating a method for assembling thebone fixation subassembly illustrated in FIGS. 14A-B;

FIG. 16B is a schematic view illustrating the method for assembling thebone fixation subassembly illustrated in FIGS. 14A-B;

FIG. 16C is a schematic view illustrating the method for assembling thebone fixation subassembly illustrated in FIGS. 14A-B, and showing thecollet extension being clipped onto the bone anchor;

FIG. 16D is a schematic view illustrating a method for assembling thebone fixation subassembly illustrated in FIGS. 14A-B, and showing thebone anchor and collet extension attached to the anchor seat and collet;

FIG. 17 is a perspective view of a bone anchor installed in a collet inaccordance with another alternative embodiment;

FIG. 18 is a perspective view a bone fixation subassembly constructed inaccordance with another embodiment, including the anchor and collet ofFIG. 17 installed in an anchor seat, with a portion cut away;

FIG. 19A is a sectional side elevation view of the bone fixationsubassembly illustrated in FIG. 18, taken along line 19A-19A;

FIG. 19B is a sectional side elevation view of the bone fixationsubassembly illustrated in FIG. 18, taken along line 19B-19B;

FIG. 20 is a sectional side elevation view similar the bone fixationsubassembly of FIG. 19B, showing pivotal movement of the anchor seatrelative to the bone anchor in the sagittal plane;

FIG. 21A is a sectional side elevation view similar to FIG. 19A, but ofa bone fixation subassembly constructed in accordance with anotheralternative embodiment;

FIG. 21B is a sectional side elevation view similar to FIG. 19B, but ofthe bone fixation subassembly illustrated in FIG. 21A;

FIG. 22 is a perspective view of a collet of the bone fixationsubassembly illustrated in FIG. 21A;

FIG. 23 is a perspective view of a collet of a bone fixation subassemblyconstructed in accordance with another embodiment;

FIG. 24A is a schematic view illustrating a method for assembling thebone fixation subassembly illustrated in FIG. 23;

FIG. 24B is a schematic view illustrating the method for assembling thebone fixation subassembly illustrated in FIG. 23, and showing the boneanchor being inserted into the lower end of the anchor seat therebypopping the collet over the anchor head so as to attach the anchor tothe collet;

FIG. 24C is a schematic view illustrating a method for assembling thebone fixation subassembly illustrated in FIG. 23, and showing a downwardforce being applied on the anchor relative to the anchor seat to therebybring the collet to an intermediate insertion position;

FIG. 24D is a schematic view illustrating a method for assembling thebone fixation subassembly illustrated in FIG. 23, and showing the colletbeing rotated until the flanges are brought into alignment with therecesses;

FIG. 24E is a schematic view illustrating a method for assembling thebone fixation subassembly illustrated in FIG. 23, and showing the colletpositioned radially such that the dimples are disposed in the gaps sothat the collet is unable to freely rotate or pivot relative to theanchor seat; and

FIG. 25 is a sectional top plan view of the bone fixation subassemblyillustrated at step 5 of FIG. 24E.

DETAILED DESCRIPTION

Certain terminology may be used in the following description forconvenience only and should not be considered as limiting in any way.For instance, a bone fixation assembly 20 includes one or more bonefixation elements 22, and four bone fixation elements 22A-D asillustrated in FIG. 1A. As shown in FIG. 1B, each bone fixation element22 extends vertically along an axial direction A, and generallyhorizontally along a radial direction R extends perpendicular to theaxial direction A. Thus, the radial direction R includes a longitudinaldirection L and a lateral direction LA that extends perpendicular to thelongitudinal direction L. It should be appreciated that the directionalterms “longitudinal,” “lateral,” can likewise apply to the bone fixationassembly 20 as extending horizontally, and the directional term“transverse” can refer to a vertical direction. The bone fixationelement 22 defines an upper end 21 and a lower end 23, such that thedirectional terms “upper” and “lower” and derivatives thereof refer to adirection from the lower end 23 towards the upper end 21, and from theupper end 21 towards the lower end 23, respectively.

The words “inward,” “outward,” “upper,” “lower,” “distal,” and“proximal,” refer to directions toward or away from, respectively, thegeometric center of the bone fixation assembly 20 and its components.The words, “anterior”, “posterior”, “superior,” “inferior” and relatedwords and/or phrases designate preferred positions and orientations inthe human body to which reference is made and are not meant to belimiting. It should further be appreciated that while round structuresdefine diameters as described herein, the round structures could bereplaced with alternative (e.g., polygonal) structures which woulddefine alternative cross-sectional dimensions opposed to diameters. Theterm “diameter” as used herein is intended to include all suchalternatives unless otherwise specified. The terminology includes theabove-listed words, derivatives thereof and words of similar import.

It should be appreciated that the directional terms are used herein withreference to the orientation of the bone fixation assembly 20 and itscomponents as illustrated, and that the actual orientation of the bonefixation assembly 20 and its components may change during use. Forinstance, the axial direction is illustrated as extending along avertical direction, and the radial direction is illustrated as extendingalong a horizontal direction, however the directions that encompass thevarious directions may differ during use, depending, for instance, onthe desired orientation of the bone fixation assembly 20 during use.Accordingly, the directional terms are used herein merely for thepurposes of clarity and convenience only, in a non-limiting manner.

Referring now to FIG. 1A, the bone fixation assembly 20 includes aplurality of bone fixation elements, such as bone fixation elements22A-D, connected by a fixation rod 24 that extends along a longitudinalaxis L. The bone fixation elements 22A-D each include a bone anchor 30that is implanted (e.g., screwed) into a corresponding vertebra 27A-D.Unless otherwise specified, the bone fixation assembly 20 and itscomponents can be made from titanium-aluminum-niobium alloy (TAN),implant-grade 316L stainless steel, or any suitable alternativeimplant-grade material.

With continuing reference to FIG. 1A, the bone fixation elements 22A-Dwill be described as and may be generally implanted in the spine, forinstance at the pedicle portion of a lumbar, thoracic, or cervicalvertebral body. In this regard, when the bone fixation elements 22A-Dare joined by the rod 24, the assembly 20 fixes the relative position ofthe vertebrae (illustrated schematically at 27A-D). Accordingly, thebone fixation elements 22A-D can be referred to as spine fixationelements or pedicle screw assemblies, the fixation rod 24 can bereferred to as a spinal rod, and the bone fixation assembly 20 can bereferred to as a spine fixation assembly. However, it should beappreciated that the bone fixation assembly 20 can also be used forfixation of other parts of the body, such as joints, long bones, orbones in the hands, face, feet, extremities, cranium, and the like.

As shown in FIG. 2, the fixation rod 24 is elongate along a longitudinalaxis L, and includes a body 25 that is cylindrical or tubular in shape.The longitudinal axis L extends generally in a cranial-caudal direction,or in the sagittal plane, when the bone fixation assembly is affixed tothe spine. The rod body 25 may include, but is not limited to, a solidbody, a non-solid body, a flexible or dynamic body, or the like, and canassume any alternative shape as desired. It should thus be appreciatedthat the bone fixation assembly 20 is not limited in use to anyparticular fixation rod 24.

Referring now to FIG. 1B, the bone fixation elements 22A-D of the bonefixation assembly 20 will now be described with respect to the bonefixation element 22. In particular, the bone fixation element 22generally includes a bone fixation subassembly 75, and a locking cap 34.The subassembly 75 is illustrated as including a bone anchor seat 26, acollet 28 disposed inside the anchor seat 26, a bone anchor 30 (shown asa threaded bone screw) having a head portion 33 (see FIG. 3) attached tothe collet 28. The locking cap 34 is installed in the anchor seat 26 ata location above the collet 28, such that the fixation rod 24 is locatedin a rod slot 36 that is disposed, and as illustrated defined, betweenthe collet 28 and the locking cap 34. As will be appreciated from thedescription below, the bone anchor 30 is free to pivot with respect tothe anchor seat 26 in a desired plane, which can be the sagittal plane,and can further freely rotate relative to the anchor seat 26. Becausepivotal motion of the bone anchor is limited to the desired plane, thebone fixation elements 22 can be referred to as a uni-planar bonefixation element, and the bone fixation assembly can be referred to as auni-planar bone fixation assembly.

Referring also to FIG. 3, the bone anchor 30 is configured as a bonescrew, or pedicle screw, that includes an externally threaded shaft 31coupled at its upper end to an enlarged curved head 33. The shaft 31extends axially along a central axis B of rotation, and can define anysuitable diameter, length, and thread design so as to engage theunderlying bone, such as a vertebra 27. Alternatively, the shaft 31 canbe unthreaded so as to define a pin or a nail if desired. Thus, oneskilled in the art will appreciate that the bone anchor 30 is notlimited to any particular type of shaft 31. The bone anchor 30 may alsobe cannulated and fenestrated such that openings extend radially outwardfrom a central hollow channel in a cannulated shaft to urge fluid out ofthe bone anchor 30 during injection or draw fluid into the centralhollow channel from the radial sides of the anchor during extraction ofmaterial adjacent the anchor if desired.

The bone anchor 30 further includes a vertically extending neck 35connected between the shaft 31 and the head 33. The neck 35 isillustrated as extending axially in a direction parallel to axis B, andincludes an outer neck surface 37 that defines a neck diameter, which isless than the diameter of the head 33.

The head 33 can define a semi-spherical curvature, or can alternativelydefine any suitable curvature as desired to facilitate rotation withrespect to the collet 28 as is described in more detail below. The head33 defines a pivot location that extends along a lateral pivot axis ofrotation LA_(P) that extends through the head in a direction parallel tothe lateral axis LA. The head 33 further defines a pivot location thatextends along a longitudinal axis of rotation L_(P) that extends throughthe head in a direction parallel to the longitudinal axis L. The head 33also includes a drive surface 39 configured to receive a correspondingtip of a drive tool, such as a screw driver configured to rotate thebone anchor 30 into engagement with the vertebrae 27 or other underlyingbone surface. The drive surface 39 can define a hexagon, a star drivepattern, a Phillips head pattern, a slot for a screw driver, threadsconfigured to receive corresponding threads of a threaded drive post, orany suitable drive tool engaging structure as desired.

Referring now to FIG. 4A, the anchor seat 26 includes an anchor seatbody 38 that can be described as a generally cylindrical tubular bodyextending centrally along an axial axis A that extends generally in theanterior-posterior direction in the sagittal plane when the bonefixation element is implanted in the underlying vertebra. Thus, as knownby those having ordinary skill in the art, the longitudinal axis L andthe axis A extend substantially in the sagittal plane when the bonefixation assembly 20 is fixed in the vertebrae 27. The body 38 includesa base 40 and a pair of spaced opposing arms 42 extending out (up inillustrated the orientation) from the base 40. The arms 42 can besubstantially identically or identically constructed. The arms 42 definecorresponding upper ends 46 that are also the upper ends of the body 38,and define an upper opening 48. The base 40 defines a lower end 50 thatis also the lower end of the body 38, and defines a lower opening 52.The body 38 defines an axial bore 54 extending from the lower opening 52to the upper opening 48.

The body 38 includes a pair of spaced opposing support walls 56 and apair of spaced opposing spacer walls 58 connected between the supportwalls 56. The support walls 56 can be substantially identically oridentically constructed, and the spacer walls 58 can likewise besubstantially identically or identically constructed. The arms 42 extendup from respective support walls 56, and can be shaped as desired. Asillustrated, the arms 42 are arc-shaped with the axis of the arc passingthrough the plane of symmetry that bisects the anchor seat 26. Each arm42 extends circumferentially about its axis less than 180°, such asbetween 60° and 150°, for instance approximately 90°. In one highlypreferred embodiment, each arm 42 extends circumferentially 90.5° aboutits axis. Accordingly, a gap G extends circumferentially betweenadjacent circumferentially outer ends of the arms 42. The opposing gapsG are in alignment with the axial bore 54. The arms 42 can be disposedradially opposite each other such that the gaps G, in combination withthe aligned portion of the axial bore 54, define a rod-receiving channel36 that is sized and configured to receive the fixation rod 24 such thatthe fixation rod 24 extends through the bone fixation element 22. Thus,the gaps G are aligned in the longitudinal direction. The fixation rod24 can thus extend through the opposing gaps G and the axial bore 54.The arms 42 define radially inner and outer surfaces 60 and 62,respectively. The inner surfaces 60 define threads 60A, and areconfigured to threadedly receive the locking cap 34, as will now bedescribed.

In particular, referring to FIGS. 5A-C, the locking cap 34 isillustrated as a set screw 64 and a saddle 66 operatively coupled to theset screw 64. The set screw 64 includes a generally cylindrical setscrew body 65 having external threads 68 configured to threadedly engagethe threads 62 formed on the inner surfaces 60 of the arms 42. Inaccordance with one embodiment, the threads 68 and 62 can incorporateinclined load flanks forming an angle with respect to the axis A of thebone fixation element 22. The load flanks may converge so that the topsurface of the thread and the bottom surface of the thread converge. Theangle may be between 0 degrees (0°) and 30 degrees (30°), and in oneembodiment can be about five degrees (5°). One skilled in the art willappreciate that the threads may take on any alternative form as desired,including negative load threads, perpendicular threads, buttressthreads, or the like.

The externally threaded set screw 64 generally provides flexibility wheninserting the fixation rod 24 into the anchor seat body 38 such that thefixation rod 24 need not be completely reduced or seated within the body38 prior to engagement of the locking cap 34. The set screw 64 isconfigured to be tightened within the anchor seat 26 against thefixation rod 24. The locking cap 34 may be constructed as desired forthis purpose including, but not limited to, an externally threaded cap,a quarter-turn or partial-turn locking cap, a two-piece screw set, orthe like.

The set screw 64 is illustrated as including a drive surface 70 providedas an internal recess extending vertically down into the upper end ofthe screw 64. The drive surface has any suitable shape configured tocooperate with a corresponding drive tool for threadedly securing theset screw 64 onto the anchor seat body 38. The drive surface 70 candefine any shape as desired, for instance an external hexagon, a stardrive pattern, a Phillips head pattern, a slot for a screw driver, athreading for a correspondingly threaded post, or the like.

With continuing reference to FIGS. 5A-C, the saddle 66 includes a saddlebody 72 having a transverse recess 74 extending up into the bottom endof the saddle body 72. The recess 74 can define a round surface thatextends about a longitudinally extending axis, such that the recess 74is configured to receive the fixation rod 24 at a rod-contacting surface76. The rod-contacting surface 76 can include a desired surface finishthat adds roughness, such as, for example, a knurl, bead blasting,grooves, or other textured finish that increases surface roughness andenhances rod push through strength.

The saddle 66 can be coupled to the set screw 64 in any desired manner,including adhesion, mechanical fastening, and the like. In theillustrated embodiment, the saddle 66 includes a stem 78 extendingcentrally upward from the saddle body 72. The stem 78 is configured tobe received in a central bore 32 extending vertically into the lower endof the set screw body 65, and can be fastened within the central borewith a rivet 80 or other like fastener. Accordingly, the saddle 66 isrotatable relative to the set screw 64, such that the saddle 66 canself-align with the fixation rod 24 as the set screw 64 is being rotatedwith respect to the anchor seat 26, for instance when the locking cap 34is being tightened against the fixation rod 24.

Referring again to FIG. 4A, and as described above, the anchor seat body38 includes a pair of spaced opposing support walls 56 and a pair ofspaced opposing spacer walls 58 connected between the support walls 56.The arms 42 extend up from respective support walls 56, such that thespacer walls 58 are disposed between the arms 42. Each of the spacerwalls 58 defines opposing upper ends 84 and lower ends 82 that can beshaped as desired. The upper ends 84 are round in accordance with theillustrated embodiment, such that the upper ends 84 and thecircumferentially outer ends of the arms 42 are adjoined to generallydefine a U-shape from a horizontal view through the gaps G. Thus, theupper ends 84 define the lower end of the gaps G.

The upper ends 84 can be shaped to conform generally with the outersurface of the fixation rod 24, such that the upper ends 84 receive andengage the fixation rod 24 during use. Alternatively, the upper ends 84can be spaced slightly below the upper surface of the collet 28, suchthat the collet 28 supports the fixation rod 24 during use, as will bedescribed in more detail below.

The support walls 56 each define opposing inner and outer surfaces 86and 88, respectively. The support walls 56 flare inward toward axis A ina downward direction from the arms 42, and terminate at respective lowerends 90. The inner surfaces 86 of each support wall 56 at the lower end90 define a distance D therebetween that is less than the distancebetween opposing radially opposing inner surfaces 60 of the arms 42, andgreater than the diameter of the head 33 of the bone anchor 30. Theinner surfaces 86 flare radially inward toward the central axis A, andtoward each other, along a downward direction, and are each connected tobottommost, and innermost, surfaces that define respective longitudinalguide walls 92.

Referring also to FIG. 4B, each guide wall 92 defines respective innerguide surfaces 93 that extend in a desired plane through which the boneanchor 30 is permitted to pivot relative to the anchor seat 26. In theillustrated embodiment, the desired plane is the sagittal plane SP. Itshould be appreciated in alternative embodiments that the anchor seat 26can be constructed such that the guide walls 92 extend in any desiredalternative plane, such as a plane defined by the medial-lateral andanterior-posterior directions.

Referring also to FIG. 7A, the opposing guide walls 92 define a distancetherebetween that is substantially equal to the diameter of the neck 35,such that the guide walls 92 are configured to abut opposing abutmentsurfaces of the bone anchor, which are illustrated as opposing sides ofthe outer neck surface 37 when the bone anchor 30 is disposed in theanchor seat 26. The opposing sides of the outer neck surface 37 arespaced along a lateral axis that is perpendicular to the longitudinalaxis L. Accordingly, the guide walls 92 prevent the bone anchor 30 frompivoting along a first direction D toward either guide wall 92 withrespect to the anchor seat 26, for instance about the longitudinal pivotaxis L_(P). Thus, the guide walls 92 provide a guide or track thatpermits the bone anchor 30 to pivot along the guide or track only in adesired plane that is defined by the guide wall 92 (e.g., sagittalplane) and includes the axis A, and prevents the bone anchor 30 frompivoting in any other plane that 1) includes the axis A, and 2)intersects the desired plane. When the anchor 30 pivots in the desired(e.g., sagittal) plane, the axis of rotation B becomes angularly offsetwith respect to the central axis A of the subassembly 75.

Alternatively, the guide walls 92 can be spaced apart a distance greaterthan the diameter of the neck 35, but disposed within close proximity ofthe bone anchor 30, so as to limit pivotal movement of the bone anchor30 relative to the anchor seat 34 in the first direction D in a planethat is perpendicular to the desired (e.g., sagittal plane), forinstance about the longitudinal pivot axis L_(P). The bone anchor 30 canpivot about the longitudinal pivot axis L_(P) through a range of anglesdefined by the central anchor axis B and the central anchor seat axis Athat is less than 10°, for instance less than 5°, such as 0°. Thedistance between the opposing guide walls 92 is less than the diameterof the head 33 of the bone anchor 30, while the inner surfaces 86 definea distance therebetween that is slightly greater than the diameter ofthe anchor head 33.

Referring now to FIGS. 4 and 7B, the lower ends 82 of the spacer walls58 are connected between the outer ends of the opposing support walls56, and extend in a direction substantially perpendicular to the guidewalls and perpendicular to the longitudinal axis L. Furthermore, thelower ends 82 are upwardly displaced with respect to the guide walls 92.Accordingly, the lower ends 82 are positioned to permit the bone anchor30 to pivot in a second direction E about the lateral pivot axis LA_(P)along a second plane (e.g., the sagittal plane) relative to the anchorseat 26 until the neck 35 of the bone anchor 30 abuts one of the lowerends 82. In this regard, the lower ends 82 can be referred to as stops.Because the lower ends 82 are displaced above the guide walls 92, thebone anchor can pivot more in the second direction E than in the firstdirection D. For instance, as illustrated, the bone anchor 30 can pivotsuch that the central axis B of the bone anchor can be angularly offsetwith respect to the central axis A of the anchor seat 26 through a rangeof angles+/−between 0° and 90° with respect to the vertical, such asbetween 5° and 45°, including between 15° and 35°, for instance 25° inthe sagittal plane before the neck 35 abuts the lower ends 82. The neck35 of the anchor 30 can ride along the guide wall 92 as it pivots in thesagittal plane SP.

Referring now to FIG. 6, the collet 28 includes a collet body 45 thatdefines a first or upper end 47 sized and configured to contact orsupport at least a portion of the fixation rod 24 when the rod isreceived within the rod-receiving channel 36, and a second or lower end49 sized and configured to contact or otherwise engage, directly orindirectly, a portion of the bone anchor head 33. The collet body 45 isannular, and thus defines an axial bore 53 extending between and throughthe upper and lower ends 47 and 49. The axial bore 53 is aligned withthe axial bore 54 when the collet 28 is installed in the anchor seat 26.

Referring to FIGS. 6 and 7A-B, the upper end 47 defines radiallyopposing upwardly facing seat portions 51 having a curvature orsemi-spherical shape corresponding to the outer surface of the fixationrod 24, and is therefore configured to receive or otherwise support atleast a portion (e.g., a lower portion) of the rod 24. The lower end 49defines an inner surface 55 defining a curvature or semi-spherical shapecorresponding to the outer surface of the anchor head 33, and istherefore configured to receive or otherwise engage at least a portionof the head 33, so that the head can rotate with respect to the collet28 and the anchor seat 26, and can further pivot with respect to thecollet 28 as permitted by the anchor seat 26. Because the bone anchor 30can freely rotate about its axis of rotation B relative to the anchorseat 26, and thus the anchor seat 26 can likewise rotate about the boneanchor 30, the rod-receiving channel 36 can be aligned with the fixationrod 24 without advancing or withdrawing the bone anchor 30 in or out ofthe underlying bone. Thus, the bone anchor 30 can maintain a constantinsertion depth in the underlying bone (e.g., vertebra 27) whileadjusting the orientation of the rod-receiving channel 36.

The collet 28 further includes a pair of flanges 57 extending up fromthe upper end 47 of the collet body 45 at a location radially betweenthe seat portions 51. A locking lip 59 extends radially out from eachflange 57. As best shown in FIG. 7A, the anchor seat 26 defines a pairof opposing recesses 61 formed radially in the opposing inner surfaces86 of the support walls 56 at a location below the threaded innersurfaces 60 of the arms 42. During operation, the collet 28 can beinserted down into the anchor seat 26, thereby causing the flanges 57 toflex inwardly past the threaded inner surfaces 60, until the lips 59clear the upper ends 63 of the recesses 61, at which point the flanges57 snap back out so that the lips 59 are disposed in the recesses 61.Interference between the lips 59 and the upper ends 63 prevent thecollet 28 from backing out through the upper end of the anchor seat 26.The recesses 61 further define a circumferential length substantiallyequal to that of the flanges 57 and locking lips 59, such that thecollet 28 is rotationally fixed with respect to the anchor seat 26 in aposition whereby the upper surface 47 is aligned with the fixation rod24 when the fixation rod 24 is inserted into the anchor seat 26.

The lower end 49 of the collet 28 defines an outer diameter that isgreater than the inner distance between the guide walls 92. Accordingly,the collet 28 is unable to pass axially down through the lower end ofthe anchor body 26. The lower end 49 includes one or more slots 67(illustrated as a plurality of slots) extending radially therethrough soas to define opposing pluralities of fingers 69A and 69B. When thecollet 28 is disposed in the anchor seat 26 such that the lips 59 aredisposed in the respective recesses 61, the fingers 69A are axiallyaligned with the guide walls 92, and the fingers 69B are axially alignedwith the lower ends 82 of the spacer walls 58. Thus, as shown in FIGS.7A-B, when the collet 28 and anchor 30 are installed in the anchor seat26, the fingers 69A and 69B radially expand to conform with the outersurface of the anchor head 33 and the inner surfaces of the supportwalls 56 and spacer walls 58, respectively, as illustrated in FIGS.7A-B. The inner diameters defined by the opposing fingers 69A and 69Bare less than the outer diameter of the anchor head 33 to prevent theanchor 30 from being removed from the anchor seat 26 in an axiallydownward direction.

The lower ends of the fingers 69A terminate at a location above theguide walls 92, and the lower ends of the fingers 69B terminate at alocation above the lower ends 82. Accordingly, the fingers 69A and 69Bdo not interfere with the engagement between the anchor neck 35 and theguide walls 92 and lower ends 82, and thus do not interfere with thepermissible movement of the bone anchors 30 relative to the anchor seat26. Alternatively, one or both of the pluralities of fingers 69A-B couldextend below the anchor seat 26, and thus abut the anchor 30 in themanner described above with respect to the guide 92 and stop surface 82so as to direct movement of the bone anchor 30 in a desired direction(e.g., pivot in the sagittal plane).

Referring now to FIGS. 8A-8D, a method for assembling a bone fixationsubassembly 75 includes at step 1, inserting the bone anchor 30vertically down through the axial bore 54, such that the shaft 31extends through the lower opening 52 of the lower end 50 of the anchorseat 26. Next, at step 2, the collet 28 is inserted into the axial bore54 to a location whereby the locking lips 59 can engage the lowermostthreads 62 of the inner surface 60 of the arms 42. Next, at step 3, anupward force can be applied to the bone anchor 30 so as to insert theanchor head 33 into the lower end 49 of the collet 28. The locking lips59 of the collet 28 brace against the anchor seat 26 inside the threads62 to prevent the upward force applied by the screw 28 from causing thecollet 28 to back out of the upper opening of the anchor seat 26. Atstep 4, a downward force is applied to the collet 28, thereby insertingthe locking lips 59 into the recesses 61 in the manner described above,and locking the anchor 30 and collet 28 in the anchor seat 26.

It should thus be appreciated that the subassembly 75 can include thecollet 28 installed in the anchor seat 26, and the bone anchor 30installed in the collet 28. In alternative embodiments, a subassemblycan be provided include the collet installed in the anchor seat withouta bone anchor installed in the collet. In these embodiments, the boneanchor can be implanted into underlying bone before the anchor head isinserted into the collet. The anchor 30 can comprise a pin or nail, or ascrew as desired. It should be appreciated that the bone fixationsubassembly 75 and the alternative bone fixation subassemblies describedherein can likewise be referred to as spine fixation subassemblies when,for instance, they are configured for implantation into one or morevertebrae for vertebral fixation.

During use, because the bone anchor 30 is rotatable with respect to thecollet 28 and the anchor seat 26, a driving tool can engage the drivesurface 39 of the head 33 so as to insert the threaded shaft 31 into theunderlying bone, as shown in FIG. 1A. Next, as shown in FIGS. 7A-D, theanchor seat 26 can be rotated about axis A in the direction of Arrow Rabout the full 360° range of angles so as to align the rod-receivingchannel 36 with the longitudinal axis of the fixation rod 24. Once thebone anchor 30 has reached a desired depth in the underlying vertebra,the fixation rod 24 can be inserted into the subassembly 75. Inparticular, the fixation rod 24 is inserted into the axial bore 54either horizontally through the gaps G, or vertically down into theaxial bore 54. It should be appreciated that the fixation rod 24 will beseated in the upper end 47 of the collet 28.

With continuing reference to FIGS. 7A-D, once the rod 24 is installed inthe subassembly 75, the locking cap 34 can be attached to thesubassembly 75 so as to fully assembly the anchor assembly 22. In thisregard, it should be appreciated that the subassembly 75 can furtherinclude the fixation rod 24 and/or the locking cap 34. In theillustrated embodiment, the external threads 68 of the set screw 64 arerotated within the inner threads 62 of the anchor seat arms 42, therebycausing the set screw and saddle 66 to move axially down in the axialbore 54. As the saddle 66 approaches the fixation rod 24, the saddle 66is rotated with respect to the set screw 64 so as to bring therod-contacting surface 76 into alignment with the fixation rod 24. Oncethe saddle 66 is aligned with the fixation rod 24, the set screw 64 iscontinuously threadedly inserted into the bone anchor 26, such that thelocking cap 34 can be tightened against the rod 24, thereby applying adownward axial force to the rod 24. The locking cap 34 can be said to bein an initial position when installed in the locking cap 34 but beforeapplying an axial force against the fixation rod 24. The axial forceapplied to the rod 24 by the locking cap 34 is transmitted to the collet28, which causes the fingers 69A to ride along the inner surfaces 86 ofthe support wall 56, and fingers 69B to ride along the radially innersurfaces of the spacer walls 58.

As the fingers 69A ride along the inner surfaces 86, they becomeradially inwardly displaced due to the inward flare of the innersurfaces 86, thereby radially biasing, or radially compressing, thefingers 69A against the anchor head 33. Likewise, as the fingers 69Bride axially down along the radially inner surfaces of the spacer walls58, the fingers 69B become disposed between the anchor head 33 and thespacer walls 58, thereby radially compressing the fingers 69B againstthe anchor head 33. Increasing radial compression of the fingers 69A-Bagainst the anchor head 33 causes frictional forces between the fingers69A-B and the anchor head 33 that resist both rotation of the anchor 30about the axis A relative to the anchor seat 26, collet 28, and fixationrod 24, and also resist pivoting of the anchor, for instance in thesagittal plane. When the locking cap is fully tightened to a lockedposition, the resulting frictional forces prevent the anchor 30 frommovement relative to the anchor seat 26, collet 28, and fixation rod 24.Thus, the locking cap 34 is configured to transmit a locking force ontothe collet 28 and bone anchor 30 to fix or lock the position of the boneanchor 30 relative to the anchor seat 26 and fixation rod 24.Furthermore, when the locking cap 34 is in the locked position, thefixation rod 24 is captured between the saddle 66 and the upper surfaceof the collet 28 such that the anchor seat 26 is prevented from movementrelative to the fixation rod 24.

It should be appreciated that any time after the bone anchor 30installed in the anchor seat 26 and before the bone anchor 30 is lockedin place, the bone anchor 30 can be pivoted in the sagittal plane SPabout the lateral pivot axis LA_(P) in the manner described above, whilethe bone anchor 30 is prevented from pivoting in all other planes.Because the guide walls 92 prevent the bone anchor 30 from pivotingabout an axis that is angularly offset with respect to the lateral pivotaxis LA_(P) in the illustrated embodiment, the surgeon is assured thatthe anchor 30 can only pivot in the sagittal plane SP, and will notcause the vertebrae 27 to become misaligned due to movement of the boneanchor 30 in a direction other than in the sagittal plane. Accordingly,the bone fixation assembly 20 incorporating the bone fixation elements22 has particular utility in addressing spinal misalignments such as therotational component of a scoliosis deformity.

In one particular method, the locking cap 34 can be tightened againstthe rod to an intermediate position that sufficiently radiallycompresses the fingers 69A-B against the bone anchor 30 that preventsthe bone anchor 30 from freely pivoting in the sagittal plane, forinstance under gravitational forces, while allowing a surgeon topivotally adjust the angular position of the bone anchor 30 in from aninitial position in a desired plane to a desired position in the desiredplane by applying a force to the bone anchor 30 that overcomes thefriction between the bone anchor 30 and the fingers 69A-B. In theillustrated embodiment, the desired plane is the sagittal plane Once thebone anchor 30 is in the desired angular position in the sagittal plane,the locking cap 34 can be further tightened to a locked position wherebythe bone anchor 30 is locked in place in the desired angular position inthe desired plane. It should be appreciated that the end cap 34 can beunthreaded from the locked position into the intermediate position orthe initial position if it is desired to further adjust the angularposition of the bone anchor 30.

It should be appreciated that the above-described method steps can beperformed for each bone fixation element of the bone fixation assembly20 as desired. Furthermore, the method steps described above need not beperformed in the order described, and it should be appreciated that thatone or more of the steps can be omitted if desired. It should be furtherappreciated that while the guide walls 92 prevent the bone anchor 30from pivoting in a first plane that intersects the sagittal planerelative to the anchor seat 26 and fixation rod 24, the guide walls 92could alternatively be slightly spaced with respect to the neck 35, suchthat the anchor 30 can pivot about a first plane that intersects thesecond sagittal plane within an angular range that is less than theangular range that the anchor 30 can pivot in the sagittal plane.

Referring to FIGS. 1-8 generally, one or more bone fixation assemblies20 can be provided as one or more bone fixation elements 22 provided asa bone fixation kit. The kit can include at least one of the followingcomponents: one or more bone anchors 30, one or more locking caps 34,one or more collets 28, one or more fixation rods 24, one or more anchorseats 26, and/or one or more preassembled bone fixation subassemblies75, including a bone anchor 30 and collet 28 pre-installed in the anchorseat 26 in the manner described above. Thus, the surgeon can implant thebone anchor 30 of a plurality of subassemblies in an underlying bone,such as a vertebra, and the anchor seats of the subassemblies can becoupled to a fixation rod 24 in the manner described above. It shouldfurther be appreciated that the components included in the kit may varyby at least one characteristic. For example, the components included inthe kit can vary in size and shape, and/or they can be constructed inaccordance with alternative embodiments as described herein, or theycould be identically constructed with the same size and shape. Forinstance, fixation rods 24 and bone anchors 30 can be provided havingdifferent diameters and lengths, and the bone anchors 30 can be providedas screws and nails or pins. Alternatively, several kits can beprovided, each individual kit including components corresponding to aparticular size, and shape, and embodiment, wherein the size, shape,and/or embodiment of the components of the kits can vary from kit tokit.

While the bone fixation element 22 has been illustrated and described inaccordance with one embodiment, it should be appreciated that bonefixation elements, a bone fixation assembly incorporating the bonefixation elements, and bone fixation kits can be constructed inaccordance with several alternative embodiments.

For instance, referring to FIGS. 9-10, a bone fixation subassembly 175constructed in accordance with an alternative embodiment is illustratedincluding reference numerals corresponding to like elements of the bonefixation subassembly 75 incremented by 100. Thus, the bone fixationsubassembly 175 can be constructed as described with respect to the bonefixation subassembly 75 except as otherwise noted. It should beappreciated that one or more up to all of the bone fixation elements ofthe bone fixation assembly 20 can include the bone fixation elements 22or alternative bone fixation elements as described herein, one or moreof which can include the bone fixation subassembly 75, the bone fixationsubassembly 175, or any alternative bone fixation subassembly asdescribed herein.

The bone fixation subassembly 175 includes an anchor seat 126, and acollet 128 preinstalled in the anchor seat 126. The subassembly 175 canfurther include a bone anchor 30 preinstalled in the collet 128, whichin turn is preinstalled in the anchor seat 126. Alternatively, thesubassembly 175 can include the collet 128 preinstalled in the anchorseat 126, and the bone anchor 30 can be later installed as desired.

The anchor seat 126 includes an anchor seat body 138 extending centrallyalong the central axis A. The body 138 includes a base 140 and a pair ofspaced opposing arms 142 extending up from the base 140. The base 140defines a lower end 150 that is also the lower end of the body 138, anddefines a lower opening 152. The body 138 defines an axial bore 154extending from the lower opening 152 to the upper opening 148.

The arms 142 extend up from respective support walls 156, and theopposing spacer walls 158 are connected between the support walls 156.The arms 142 define internal threads 162 that are configured to engagethe external threads 68 of the locking cap 34 as described above. Thearms 142 further define gaps G therebetween that are configured toreceive the fixation rod 24 as described above. The support walls 156flare radially inward toward axis A in a downward direction from thearms 142 toward the lower end 150. The lower ends 150, in combinationwith the collet 128, limit pivotal movement of the bone anchor 30 to asingle plane, such as the sagittal plane, as described above. The boneanchor 30 can also rotate about its central axial axis B relative to thecollet 128 and anchor seat 126 in the manner described above.

The collet 128 includes a collet body 145 that defines a first or upperend 147 sized and configured to contact or support at least a portion ofthe fixation rod 24 when the rod is received within the rod-receivingchannel 136, and a second or lower end 149 sized and configured tocontact or otherwise engage, directly or indirectly, a portion of thebone anchor head 33. The upper and lower ends 147 and 149 areconstructed as described above, except the collet body 145 includes afinger extension 173 defining the bottom end of each of the fingers169A.

Accordingly, when the collet 128 is installed in the anchor seat 126 toprovide the preassembled bone fixation subassembly 175, the fingerextensions 173 extend through the lower opening 152 of the base 140. Thefingers 169A and finger extensions 173 define inner surfaces 155 that incombination define a curvature or semi-spherical shape corresponding tothe outer surface of the anchor head 33, and is therefore configured toreceive or otherwise engage at least a portion of the head 33. The lowerends of the fingers 169B terminate at a location displaced above thelower ends of the extensions 173, and are disposed above the loweropening 152 of the base 140 when the collet 128 is installed in theanchor seat 126.

The bone fixation subassembly 175 is constructed by inserting the collet128 down through the top of the anchor seat 126 until the locking lips59 extend in the corresponding recesses 161. The fingers 169A andextensions 173, along with the fingers 169B, are placed over the head 33of the bone anchor 30, and a downward force is applied until the fingers169A-B and extensions 173 expand radially outward to capture the head 33of the bone anchor 30 therein. The threaded shaft 31 of the bone anchor30 may already be implanted into bone prior to popping the collet 128over the head 33 of the bone anchor 30. It should be appreciated thatthe diameter of the anchor shaft 31 is not limited by the diameter ofthe opening 152 of the base 140. Thus, the diameter of the anchor shaft31 can be greater than the opening 152 of the base 140. Alternatively,the diameter of the anchor shaft 31 can be smaller than that of theopening 152, such that the bone anchor 30 can be inserted down throughthe upper opening of the anchor seat 126 and installed in the collet 128in the manner described above with respect to the bone fixation element22.

The radially inner ends of the lower ends of the finger extensions 173define guide walls 192 that are spaced apart a distance substantiallyequal to the cross-sectional dimension of the anchor neck 35. The guidewalls 192 therefore abut the neck 35 when the head 33 is captured in thecollet 128. Accordingly, the guide walls 192 permit the anchor 33 topivot only in a desired plane that is parallel to the guide walls 192,or about the lateral pivot axis LA_(P) (e.g., the sagittal plane). Thepermitted angulation in the sagittal plane is limited by contact betweenthe neck 35 and the lower ends of the spacer walls 158 in the mannerdescribed above with respect to the bone fixation element 22. Once thebone anchor 30 has been installed in the subassembly 175, the boneanchor 30 is free to rotate with respect to the collet 128 and theanchor seat 126 about the central axis B as described above, which canbe coincident with the central axis A of the anchor seat 126, orangularly offset in the sagittal plane. Once the gaps G are aligned withthe longitudinal axis L of the fixation rod 24, the fixation rod 24 andlocking cap 34 can be installed in the subassembly 175 in the mannerdescribed above.

Referring now to FIGS. 11-13, a bone fixation subassembly 275constructed in accordance with an alternative embodiment is illustratedincluding reference numerals corresponding to like elements of the bonefixation subassembly 75 incremented by 200. Thus, the bone fixationsubassembly 275 can be constructed as described above with respect toone or both of the bone fixation subassemblies except as otherwisedescribed. It should be appreciated that one or more up to all of thebone fixation elements of the bone fixation assembly 20 can include thebone fixation elements 22 or alternative bone fixation elements asdescribed herein, one or more of which can include the bone fixationsubassembly 275 as will now be described.

The bone fixation subassembly 275 includes an anchor seat 226, and acollet 228 preinstalled in the anchor seat 226. The subassembly 275 canfurther include a bone anchor 230 preinstalled in the collet 228, whichin turn is preinstalled in the anchor seat 226. Alternatively, thesubassembly can include the collet 228 preinstalled in the anchor seat226, such that the bone anchor 230 can be later installed as desired.For instance, the bone anchor 230 can be installed into the subassembly275 interoperatively (e.g., after the bone anchor 630 has been affixedin a vertebrae).

The anchor seat 226 includes an anchor seat body 238 extending centrallyalong the central axis A. The body 238 includes a base 240 and a pair ofspaced opposing arms 242 extending up from the base 240. The base 240defines a lower end 250 that is also the lower end of the body 238, anddefines a lower opening 252. The body 238 defines an axial bore 254extending from the lower opening 252 to the upper opening 248. The arms242 extend up from respective support walls 256, and the opposing spacerwalls 258 are connected between the support walls 256. The arms 242define internal threads 262 that are configured to engage the externalthreads 68 of the locking cap 34 as described above. The arms 242further define gaps G therebetween that are configured to receive thefixation rod 24 as described above.

The anchor seat 226 includes an anchor seat extension 277 that isseparate from the anchor seat body 238. The extension 277 is configuredto fasten to the bone anchor 230 prior to attaching the bone anchor 230to the collet 228. As will be appreciated below, the extension 277 isprovided as a clip that can be snapped onto the neck 235 of the boneanchor 230. The anchor seat extension 277 includes a circumferentiallyextending collar 279 and a pair of opposing posts 281 extending out (orvertically up, in the illustrated embodiment, from the collar 279. Thecollar 279 extends circumferentially greater than 180° but less than360°, and defines an inner diameter substantially equal to the outerdiameter of the anchor neck 237. A pair of tabs 283 extend up from theradially inner ends of the posts 281.

The anchor seat body 238 includes a recess 285 extending in the supportwalls 256 and one of the spacer walls 258 sized to receive the anchorseat extension 277. The recess 285 is keyed to receive the posts 281 inthe support walls 256, such that the circumferential collar extendsalong one longitudinal end of the collar body but not the otherlongitudinal end. The outer diameter of the anchor seat extension 277 issubstantially equal to the outer diameter of the lower end 250 of theanchor seat body 238, such that the outer circumferential surface of theextension 277 is flush with the outer circumferential surface of theanchor seat body 238 when the extension 277 is disposed in the recess285. The outer diameter defined by the tabs 283 is substantially equalto the inner diameter of the support walls 256.

As shown in FIGS. 13A-13C, the subassembly 275 is constructed at step 1Aby installing the collet 228 in the anchor seat 226 in the mannerdescribed above. Separately, at step 1B, the anchor seat extension 277is attached to the bone anchor 230 by clipping the collar 279 over theanchor neck 235. Next, at step 2, the anchor body 238 is brought downonto the anchor head 233, which causes the anchor seat extension 277 tobe inserted into the recess 285. The recess 285 is keyed such that theextension posts 281 are aligned with the arms 242. As the anchor body238 is brought down onto the anchor head 233, the collet fingers 169Bradially expand over the anchor head 233, and pop onto the head 233. Thecollet fingers 169B are disposed circumferentially between the extensionposts 281, and the collet fingers 169A are shorter than the fingers 169Bas shown in FIGS. 12A-B.

In one embodiment, the subassembly 275 is constructed after thecompletion of step 1A, and the bone anchor 230 and anchor extension 277can be installed into the subassembly 275 at a later time, for instancebefore or after the bone anchor 230 has been implanted in a bone, suchas a vertebra. In another embodiment, the subassembly 275 is constructedafter the completion of step 2, whereby the bone anchor 230 is attachedto the collet 228. It should further be appreciated that step 1A can becompleted prior to step 1B or that step 1B can be completed prior tostep 1A.

Once the bone anchor 230 has been installed in the anchor seat 226, theanchor 230 is free to rotate relative to the anchor seat 226 about theaxis of rotation B in the manner described above. The bottom surface ofthe posts 281 abut the neck 233, and therefore provide a laterallyspaced guide members, or a laterally extending guide, 292 that allowsthe anchor 230 to pivot in the sagittal plane, while preventing theanchor 230 from pivoting in any other angle that intersects the sagittalplane. When the anchor 230 pivots in the sagittal plane, the axis ofrotation B is angularly offset with respect to the central axis A of thesubassembly 275. The extension 277 does not extend entirely around theanchor neck 235, and thus defines a gap that is disposed on onelongitudinal side of the anchor 230. Thus, the anchor 230 is free topivot toward that longitudinal side in the sagittal plane until theanchor 230 abuts the lower end of the respective spacer wall 238, whichprovides a stop for the anchor 230. The collar 279 extends along theopposing longitudinal side of the anchor 230, and can be verticallyflush with or above the bottom surface of the corresponding spacer wall238 such that the spacer wall provides a stop with respect to angularmovement of the anchor 230 in the sagittal plane. Alternatively, thecollar 279 could be disposed below the bottom surface of thecorresponding spacer wall 238 such that the collar 279 provides a stopwith respect to angular movement of the anchor 230 in the sagittalplane. Thus, the collar 279 can limit pivotal movement in one directionin the pivotal plane. Alternatively still, the radially inner surface ofthe collar 279 could be radially outwardly displaced with respect to theradial inner surface of the posts 281, such that the collar 279 does notabut the neck 235 and thus permits pivotal movement in the sagittalplane toward the collar 279.

The collar 279 can be disposed at the inferior end of the anchor seat226 when the bone fixation element is implanted, such that the anchor230 can pivot along a greater angular range toward the superior end inthe sagittal plane than toward the inferior end. Alternatively, thecollar 279 can be disposed at the superior end of the anchor seat 226when the bone fixation element is implanted, such that the anchor 230can pivot along a greater angular range toward the superior end in thesagittal plane than toward the inferior end.

Once the anchor seat 226 has been aligned with the fixation rod 24, andthe position of the bone anchor has been located in the sagittal planeas desired, the locking cap 34 can be locked in the subassembly 275 inthe manner described above.

Referring now to FIGS. 14-16, a bone fixation subassembly 375constructed in accordance with an alternative embodiment is illustratedincluding reference numerals corresponding to like elements of the bonefixation subassembly 75 incremented by 300. Thus, the bone fixationsubassembly 375 can be constructed as described with respect to one orall of the bone fixation subassemblies described above, except asotherwise noted. It should be appreciated that one or more up to all ofthe bone fixation elements of the bone fixation assembly 20 can includethe bone fixation elements 22 or alternative bone fixation elements asdescribed herein, one or more of which can include the bone fixationsubassembly 375 as will now be described.

The bone fixation subassembly 375 includes an anchor seat 326, and acollet 328 preinstalled in the anchor seat 326. The subassembly 375 canfurther include a bone anchor 330 preinstalled in the collet 328, whichin turn is preinstalled in the anchor seat 326. Alternatively, thesubassembly 375 be provided with the collet 328 preinstalled in theanchor seat 326, such that the bone anchor 330 can be later installed asdesired.

The anchor seat 326 includes an anchor seat body 338 extending centrallyalong the central axis A. The body 338 includes a base 340 and a pair ofspaced opposing arms 342 extending up from the base 340. The base 340defines a lower end 350 that is also the lower end of the body 338, anddefines a lower opening 352. The body 338 defines an axial bore 354extending from the lower opening 352 to the upper opening 348. The arms342 extend up from respective support walls 356, and the opposing spacerwalls 358 are connected between the support walls 356. The arms 342define gaps G therebetween that are configured to receive the fixationrod 24 as described above. The arms 342 further define internal threads362 that are configured to engage the external threads 68 of the lockingcap 34 as described above.

The collet 328 includes a collet extension 387 that is separate from thecollet body 345. The extension 387 is configured to fasten to the boneanchor 330 after the bone anchor 330 has been attached to the collet328. As will be appreciated below, the extension 387 is provided as aclip that can be snapped onto the neck 335 of the bone anchor 330. Whilethe bone anchor 330 is illustrated as a nail or a pin, it should beappreciated that the anchor 330 could alternatively comprise a screw.The collet extension 387 includes a circumferential collar 389 and apair of opposing posts 391 extending out (or vertically up in theillustrated orientation) from the collar 389. The posts 391 also flareradially outward along a vertically upward direction of travel. Thecollar 389 extends circumferentially greater than 180° but less than360°, defines an inner diameter substantially equal to the innerdiameter of the collet body 345, and an outer diameter substantiallyequal to the outer diameter of the collet body 345.

The collet body 345 is keyed to receive the collet extension 387 in apredetermined orientation. In particular, the fingers 369A that arealigned with the arms 342 are shorter than the remaining fingers 369B.Accordingly, the collet extension 387 can be inserted into the anchorseat 326 such that the posts 391 are circumferentially aligned with, andlocated between, the fingers 369B. The circumferential collar extendsalong one longitudinal end of the collar body but not the otherlongitudinal end.

The subassembly 375 is constructed using a method that begins at step 1)wherein the collet 328 is installed in the anchor seat 326 by insertingthe collet 328 vertically upward into the lower opening 352 of theanchor seat body 338, in the manner described above. In one embodiment,the kit can include the subassembly 375 as including the anchor seat 326and the installed collet 328. Next, at step 2, the anchor body 338 isbrought down onto the anchor head 333 (or the anchor head 333 is broughtup into the anchor body 338), thereby causing the collet fingers 169A-Bto expand radially over the head 333 and snap down over the head 333 tosecure the anchor therein. At step 3, the collet extension 387 isclipped around the neck 335, such that the posts 391 extend into a gapbetween the lower ends of adjacent fingers 369A. Thus, the collet body345 is keyed such that the posts 391 are vertically aligned with thearms 342. The subassembly 375 is thus provided at step 4.

Thus, FIGS. 16A-16D illustrates the subassembly as including the anchorseat 326 and the collet 328 installed in the anchor seat 326, the colletextension 387 clipped onto the bone anchor 330, and the bone anchor 330and collet extension 387 attached to the anchor seat 326 and collet 328.In an alternative embodiment, the subassembly 375 can include the anchorseat and the collet 328 without the anchor 330 and extension 387installed. The anchor 330 can be implanted into the underlying bone(e.g., vertebra) before or after the collet extension 387 is clippedonto the neck, and prior to or after the anchor 330 is attached to thecollet 328. In this regard it should be appreciated that the anchor 330is freely rotatable within the collet 328 and extension 387 with respectto the anchor seat 326.

The bottom surface of the posts 381 abut the neck 333, and thereforeprovide a laterally extending guide 392 that allows the anchor 330 topivot in the sagittal plane, while preventing the anchor 330 frompivoting in any other angle that intersects the sagittal plane. Theextension 377 does not extend entirely around the anchor neck 335, andthus defines a gap that is disposed on one longitudinal side of theanchor 330. Thus, the anchor 330 is free to pivot toward thatlongitudinal side in the sagittal plane until the anchor 330 abuts thelower end of the respective spacer wall 338, which provides a stop forthe anchor 330 in the sagittal plane. The collar 379 extends along theopposing longitudinal side of the anchor 330, and can be verticallyflush with or above the bottom surface of the corresponding spacer wall338 such that the spacer wall provides a stop with respect to angularmovement of the anchor 330 in the sagittal plane. Alternatively, thecollar 379 could be disposed below the bottom surface of thecorresponding spacer wall 338 such that the collar 379 provides a stopwith respect to angular movement of the anchor 330 in the sagittalplane. Thus, the collar 379 can limit pivotal movement in one directionin the pivotal plane. Alternatively still, the radially inner surface ofthe collar 379 could be radially outwardly displaced with respect to theradial inner surface of the posts 381, such that the collar 379 does notabut the neck 335 and thus permits pivotal movement in the sagittalplane toward the collar 379.

The collar 379 can be disposed at the inferior end of the anchor seat326 when the bone fixation element is implanted, such that the anchor330 can pivot along a greater angular range toward the superior end inthe sagittal plane than toward the inferior end. Alternatively, thecollar 379 can be disposed at the superior end of the anchor seat 326when the bone fixation element is implanted, such that the anchor 330can pivot along a greater angular range toward the superior end in thesagittal plane than toward the inferior end.

Once the anchor seat 326 has been aligned with the fixation rod 24, andthe position of the bone anchor 330 has been located in the sagittalplane as desired, the locking cap 34 can be locked in the subassembly375 in the manner described above.

Referring now to FIGS. 17-20, a bone fixation subassembly 475constructed in accordance with an alternative embodiment is illustratedincluding reference numerals corresponding to like elements of the bonefixation subassembly 75 incremented by 400. Thus, the bone fixationsubassembly 475 can be constructed as described with respect to one orall of the bone fixation subassemblies, except as otherwise noted. Itshould be appreciated that one or more up to all of the bone fixationelements of the bone fixation assembly 20 can include the bone fixationelements 22 or alternative bone fixation elements as described herein,one or more of which can include the bone fixation subassembly 475 aswill now be described.

The bone fixation subassembly 475 includes an anchor seat 426, and acollet 428 preinstalled in the anchor seat 426. The subassembly 475 canfurther include a bone anchor 430 that can be preinstalled in the collet428 as part of the subassembly 475 provide in the kit, or the boneanchor 430 can be provided separately, and later installed in thesubassembly 475, for instance before or after being implanted in anunderlying bone. The anchor can comprise a pin or nail, or a screw asdesired.

The anchor seat 426 includes an anchor seat body 438 extending centrallyalong the central axis A. The body 438 includes a base 440 and a pair ofspaced opposing arms 442 extending up from the base 440. The base 440defines a lower end 450 that is also the lower end of the body 438, anddefines a lower opening 452. The body 438 defines an axial bore 454extending from the lower opening 452 to the upper opening 448. The arms442 extend up from respective support walls 456, and the opposing spacerwalls 458 are connected between the support walls 456. The arms 442define gaps G therebetween that are configured to receive the fixationrod 24 as described above. The arms 442 further define internal threads462 that are configured to engage the external threads 68 of the lockingcap 34 as described above.

The collet 428 includes a collet body 445 that defines a first or upperend 447 sized and configured to contact or support at least a portion ofthe fixation rod 24 when the rod is received within the rod-receivingchannel 436, and a second or lower end 449 sized and configured tocontact or otherwise engage, directly or indirectly, a portion of thebone anchor head 33. The upper and lower ends 447 and 449 are generallyconstructed as described above, with respect to the collet 28, exceptthe collet body includes fingers 469 of the same vertical length. Ofcourse, it should be appreciated that one or more of the fingers 469 canextend down a greater or lesser difference than one or more of the otherfingers.

The collet 428 further includes a pair of radially opposing protrusions493 projecting radially inward from the upper ends of the collet flanges457. When collet 428 is installed in the anchor seat 426, the flanges457 are located in the recesses 461 such that the protrusion 493 isdisposed radially centrally with respect to the arms 442 when the collet428 is installed in the anchor seat. Each protrusion 493 defines a lowervertex 495 and upwardly angled walls 497 extending longitudinally outfrom the vertex 495. When anchor head 333 is in the collet fingers 469,the vertex 495 abuts the upper end of the anchor head 433, which can beflat in a radial direction perpendicular to the axis of rotation B.

As illustrated in FIG. 20, the protrusion 493 permits the anchor 430 topivot in a desired plane (e.g., the sagittal plane) relative to thecollet 428 and anchor seat 426, while interference between the vertex495 and the anchor head 433 prevents the anchor 430 from pivoting in anyplane that intersects the sagittal plane. In particular, the anchor 430can pivot in the desired plane through an angular range defined by theaxis A and pivoted axes of rotation B′ of the bone anchor 430. Thus, theprotrusions 493 provide a guide that permits the anchor 430 to pivot inthe desired plane while preventing pivotal movement of the bone anchorin all other planes that intersect the sagittal plane. The anchor 430can freely rotate about its axis B with respect to the collet 428 andthe anchor 426 in the manner described above.

The bone fixation subassembly 475 is constructed by inserting the collet428 down through the top of the anchor seat 426 until the flanges 457are locked in the corresponding recesses 461. The fingers 469A areplaced over the head 33 of the bone anchor 30, and a downward force isapplied against the anchor 430 until the fingers 169 expand radiallyoutward to capture the head 433 of the bone anchor 430 therein. Thethreaded shaft 431 of the bone anchor 430 may already be implanted intobone prior to popping the collet 428 over the anchor head 433.

Once the anchor seat 426 has been aligned with the fixation rod 24, andthe position of the bone anchor 430 has been located in the sagittalplane as desired, the locking cap 34 can be locked in the subassembly475 in the manner described above.

Referring now to FIGS. 21-22, a bone fixation subassembly 575constructed in accordance with an alternative embodiment is illustratedincluding reference numerals corresponding to like elements of the bonefixation subassembly 75 incremented by 500. Thus, the bone fixationsubassembly 575 can be constructed as described above with respect toone or all of the bone fixation subassemblies, except as otherwisenoted. It should be appreciated that one or more up to all of the bonefixation elements of the bone fixation assembly 20 can include the bonefixation elements 22 or alternative bone fixation elements as describedherein, one or more of which can include the bone fixation subassembly575 as will now be described.

The bone fixation subassembly 575 includes an anchor seat 526, and acollet 528 preinstalled in the anchor seat 526. The subassembly 575 canfurther include a bone anchor 530 that can be preinstalled in the collet528 as part of the subassembly 575 provide in the kit, or the boneanchor 530 can be provided separately, and later installed in thesubassembly 575, for instance before or after being implanted in anunderlying bone. The anchor 530 is illustrated as a pin or nail, thoughthe anchor 530 could alternatively be constructed as a screw.

The anchor seat 526 includes an anchor seat body 538 extending centrallyalong the central axis A. The body 538 includes a base 540 and a pair ofspaced opposing arms 542 extending up from the base 540. The base 540defines a lower end 550 that is also the lower end of the body 538, anddefines a lower opening 552. The body 538 defines an axial bore 554extending from the lower opening 552 to the upper opening 548. The arms542 extend up from respective support walls 556, and the opposing spacerwalls 558 are connected between the support walls 556. The arms 542define gaps G therebetween that are configured to receive the fixationrod 24 as described above. The arms 542 further define internal threads562 that are configured to engage the external threads 68 of the lockingcap 34 as described above.

The collet 528 includes a collet body 545 that defines a first or upperend 547 sized and configured to contact or support at least a portion ofthe fixation rod 24 when the rod is received within the rod-receivingchannel 536, and a second or lower end 549 sized and configured tocontact or otherwise engage, directly or indirectly, a portion of thebone anchor head 33. The upper and lower ends 547 and 549 are generallyconstructed as described above, with respect to the collet 428, exceptthe collet body defines a U-shaped recess 599 extending radially inwardinto the radially outer surface of each flange 557. The recess 599 isconfigured to engage mating structure of the anchor seat 226 thatprevents the collet 528 from inadvertently backing out of the anchorseat 226 during use.

The collet 528 further includes an interior threaded surface 601disposed axially above the protrusion 593, which can receivecorresponding threads of an insertion tool when assembling thesubassembly 575. The protrusion, however, extends radially inward withrespect to the protrusion 493, such that it engages the upper surface ofa flat ledge 603 extending circumferentially about the anchor head 533.It should thus be appreciated that the flat ledge 603 defines a pair ofopposing abutment surfaces that extend along a lateral axis that extendsperpendicular to the longitudinal axis L defined by the opposing gaps G.The anchor 530 can therefore pivot in a desired (e.g., sagittal) planeabout the protrusion 593 as described above. Additionally, as describedabove, the bone anchor 530 is free to rotate within the collet 528relative to the anchor seat 526 as described above.

The bone fixation subassembly 575 is constructed by inserting the collet528 down through the top of the anchor seat 526 until the flanges 557are locked in the corresponding recesses 561. The fingers 569 are placedover the head 33 of the bone anchor 30, and a downward force is appliedagainst the anchor 530 until the fingers 169 expand radially outward tocapture the head 533 of the bone anchor 530 therein. The threaded shaft531 of the bone anchor 530 may already be implanted into bone prior topopping the collet 528 over the anchor head 533. Alternatively, the boneanchor 530 can be installed in the collet 528, and subsequentlyimplanted into underlying bone. The internal threading 601 of the collet528 allows for the use of a special driver instrument having anengagement feature, e.g., a T25 feature, which can mate with thescrewdriver to further stabilize the bone anchor 530 as it is implantedinto the underlying bone.

Once the anchor seat 526 has been aligned with the fixation rod 24, andthe position of the bone anchor 530 has been located in the sagittalplane as desired, the locking cap 34 can be locked in the subassembly575 in the manner described above.

Referring now to FIGS. 23-25, a bone fixation subassembly 675constructed in accordance with an alternative embodiment is illustratedincluding reference numerals corresponding to like elements of the bonefixation subassembly 75 incremented by 600. Thus, the bone fixationsubassembly 675 can be constructed as described above with respect toone or all of the bone fixation subassemblies, except as otherwisenoted. It should be appreciated that one or more up to all of the bonefixation elements of the bone fixation assembly 20 can include the bonefixation elements 22 or alternative bone fixation elements as describedherein, one or more of which can include the bone fixation subassembly675 as will now be described.

The bone fixation subassembly 675 includes an anchor seat 626, and acollet 628 preinstalled in the anchor seat 626. The subassembly 675 canfurther include a bone anchor 630 that can be preinstalled in the collet628 as part of the subassembly 675 provide in the kit, or the boneanchor 630 can be provided separately, and later installed in thesubassembly 675, for instance before or after being implanted in anunderlying bone. The anchor 630 is illustrated as a pin or nail, thoughthe anchor 530 could alternatively be constructed as a screw.

The anchor seat 626 extends centrally along the central axis A, andincludes a pair of spaced opposing arms 642 extending up from respectivesupport walls 656. The opposing spacer walls 658 are connected betweenthe support walls 656. The anchor seat 626 defines a lower end 650 thatdefines a lower opening 652. An axial bore 654 extends from the loweropening 652 to the upper opening 648. The arms 642 define gaps Gtherebetween that are configured to receive the fixation rod 24 asdescribed above. The arms 642 further define internal threads 662 thatare configured to engage the external threads 68 of the locking cap 34in the manner described above.

The collet 628 includes a collet body 645 that defines a first or upperend 647 sized and configured to contact or support at least a portion ofthe fixation rod 24 when the rod is received within the rod-receivingchannel 636, and a second or lower end 649 sized and configured tocontact or otherwise engage, directly or indirectly, a portion of thebone anchor head 33. The upper and lower ends 647 and 649 are generallyconstructed as described above, with respect to the collet 528, excepteach flange 657 includes a pair of opposing vertical ribs 707 spacedapart a distance by a gap 709. The outer circumferential edges of theribs 707 are spaced apart substantially the same distance as thecircumferential edges of the recess 661 formed in the inner surface ofthe support walls 656. As a result, when the collet 628 is inserted intothe anchor seat 626, the ribs 707 become disposed in the recess 661. Thecollet 628 further includes an inner threaded surface 701, andprotrusions 693 disposed below the threaded surface 701. The protrusions693 define a guide that allows the bone anchor 30 to pivot relative tothe anchor seat in the sagittal plane, in the manner described above.

The bone anchor 630 includes a pair of opposing flat ledges 703extending circumferentially around the anchor head 633 in the mannerdescribed above. The remainder of the anchor head 633 can be round inthe manner described above. Thus, the ledges 703 are configured to abutthe lower end of the protrusions 693 when the collet 628, anchor 630,and fixation rod 24 are installed in the anchor seat 626, as will now bedescribed with respect to FIGS. 24-25.

As illustrated in FIGS. 24A-24E, the bone fixation subassembly 675 isconstructed by inserting the collet 628 down through the top of theanchor seat 626 to an initial insertion position at step 1, until theflanges 657 are disposed in the corresponding recesses 661 in the mannerdescribed above. When the collet 628 is in the initial insertionposition, the upper end 647 of the collet can be aligned with the gapsG. Next, at step 2, the bone anchor 630 is inserted into the lower end652 of the anchor seat 626, thereby popping the fingers 669 over theanchor head 633 so as to attach the anchor 630 to the collet 628.Interference between the fingers 669 and the upper end of the supportwalls 656 prevents the collet 628 from backing out of the anchor seat626 in response to the upwardly directed force applied by the anchor630. The bone anchor 630 can be inserted into the anchor seat 626 andcollet 628 when assembling the subassembly 675, or after the subassembly675 has been assembled, for instance interoperatively (e.g., after thebone anchor 630 has been affixed in a vertebrae). In the positionillustrated in FIG. 2, the upper ends 647 of the collet 628 areangularly offset with respect to the rod-receiving channel 636. Asillustrated, the upper ends 647 are oriented perpendicular with respectto the channel 636.

Once the anchor head 633 is captured in the fingers 669, a downwardforce is applied on the anchor 630 relative to the anchor seat 626 atstep 3, which brings the collet 628 to an intermediate insertionposition, whereby the fingers 669 bear against the support walls 656 andspacer walls 658. In the intermediate insertion position the lower endsof the fingers 669 are aligned with the lower ends of the support walls656 and spacer walls 658. As the collet 628 moves to the intermediateinsertion position, the flanges 657 flare radially inward out ofengagement with the recesses 661, and bear against the radially innersurfaces of the support walls 656.

It should be appreciated that in the intermediate insertion position,the flanges 657 of the collet 628 are disposed in the rod-receivingchannel 636, and thus positioned to interfere with the fixation rod 24when the fixation rod is inserted. Because the flanges 657 are notdisposed in the recess 661, the collet 628, and thus the bone anchor633, is unimpeded with respect to pivotal movement about any radial axisabout a 360° range with respect to the anchor seat 626, and thereforealong any corresponding plane as desired, including the sagittal planeand any other plane angularly offset with respect to the sagittal plane.The anchor 630 is further able to freely rotate about its central axis Brelative to the collet 628 and the anchor seat 626.

At step 4, the collet 628 is rotated in the direction of Arrow A aboutaxis A, until the flanges 657 are brought into alignment with thecorresponding recesses 661. As shown in FIG. 25, the anchor seat 626includes a retention dimple 711 extending radially inward from eachsupport wall 656 at a location circumferentially centrally disposed inthe corresponding recess 661. As the collet 628 is rotated in thedirection of Arrow A, one of each pair of ribs 707 cams over the dimple711. Until each dimple 711 is disposed in the corresponding gap 709.Interference between the dimple 711 and the ribs 707 therefore resistsrotation of the collet 628 relative to the anchor seat 626. As shown instep 5 of FIG. 24E, once the collet 628 is positioned radially such thatthe dimples 711 are disposed in the gaps 709, the collet 628 is unableto freely rotate or pivot relative to the anchor seat 626. Because theprotrusions 693 are aligned with, and abut, the ledges 703 of the anchor630, the protrusions 693 provide a guide that prevents pivotal movementof the anchor 630 relative to the anchor seat 626 in all planes otherthan the sagittal plane as described above. Furthermore, when thedimples 711 are disposed in the gaps 709, the upper ends 647 of thecollet 626 are aligned with the rod-receiving channel 636, therebyproviding a seat for the fixation rod 24 in the manner described above.

In certain embodiments, when the dimples 711 are disposed in the gaps709, interference between the dimples 711 and the flanges 707 preventinadvertent rotation of the collet 628 relative to the anchor seat 626that would bring the flange out of the recess 661. However, deliberaterotation of the collet 628 about axis A relative to the anchor seat, forinstance with a tool that engages the internal threads 701, can causethe flange 657 to flex radially inward as the ribs 707 cam over thedimples 711. Once the flange 657 flexes inward, the dimple 711 is nolonger disposed in the gap 709. As the collet 628 is further rotated,the flange 657 rotates to a position circumferentially between recesses661, whereby the collet 628 and bone anchor 630 can freely pivot androtate relative to the anchor seat 626 as described above.

In this manner, in instances where the bone anchor 630 is inserted intothe underlying vertebra, and the subassembly 675 including the anchorseat 626 and collet 628 are attached to the bone anchor 630interoperatively, a surgical instrument could rotate the collet 628between its locked position whereby the dimples 711 are disposed in thegaps 709 and its unlocked position whereby the flanges 657 are disposedoutside of the recesses 661.

When the collet 628 is in the unlocked position, the bone anchor 630 ispermitted to pivot both in the sagittal plane and in any other planeangularly offset with respect to the sagittal plane about a 360° range.When the collet 628 is in the locked position, the bone anchor 630 ispermitted to pivot only in the sagittal plane relative to the anchorseat 626. Once the anchor seat 626 has been aligned with the fixationrod 24, and the position of the bone anchor 630 has been located asdesired, the locking cap 34 can be locked in the subassembly 675 in themanner described above.

It should be appreciated that the illustrations and discussions of theembodiments shown in the figures are for exemplary purposes only, andshould not be construed limiting the disclosure. One skilled in the artwill appreciate that the present disclosure contemplates variousembodiments. It should be further appreciated that the features andstructures described and illustrated in accordance one embodiment canapply to all embodiments as described herein, unless otherwiseprecluded. For instance, while the collet provides a guide in certainembodiments and the anchor seat comprises a guide in other embodiments,it should be appreciated that the above-described features of the colletcould be combined with those of the anchor seat such that at least one,or both, of the collet and anchor seat provide a guide that preventsangular movement of the bone anchor is all planes other than the desiredplane. It should be understood that the concepts described above withthe above-described embodiments may be employed alone or in combinationwith any of the other embodiments described above.

1. (canceled)
 2. A bone fixation subassembly, comprising: a bone anchorcomprising a head and a shaft that extends from the head; and an anchorseat comprising an anchor seat body that defines: an upper end thatdefines a pair of opposing fixation rod-receiving gaps that are spacedfrom one another along a longitudinal axis; a lower end that is spacedfrom the upper end along a central axis of the anchor seat body; a borethat extends from the upper end and entirely through the lower end alongthe central axis, the bore being disposed between the rod-receiving gapsand being configured to receive the head of the bone anchor such thatthe shaft of the bone anchor extends out the lower end; and a pair ofopposed guide walls spaced apart from one another on opposed sides ofthe central axis such that the opposed guide walls are configured toengage opposing sides of the bone anchor so as to permit the bone anchorto pivot relative to the anchor seat in a select plane and to preventthe bone anchor from pivoting relative to the anchor seat in any planeother than the select plane.
 3. The bone fixation subassembly of claim2, wherein the guide walls define respective inner guide surfaces thatface one another.
 4. The bone fixation subassembly of claim 2, whereinthe guide walls are spaced from one another along a first direction, andthe anchor seat defines at least one lower end surface that extendsalong the first direction, the at least one lower end surface providinga stop that limits pivoting of the bone anchor along the select plane.5. The bone fixation subassembly of claim 4, wherein the at least onelower end surface includes a pair of spaced lower end surfaces thatextend in the first direction, the lower end surfaces providing stopsthat limit pivoting of the bone anchor along the select plane on eitherside of the central axis.
 6. The bone fixation subassembly of claim 2,wherein the pair of guide walls are configured so as to permit the boneanchor to pivot in the select plane on opposite sides of the centralaxis.
 7. The bone fixation subassembly of claim 2, wherein the pair ofguide walls define a distance therebetween that is substantially equalto a distance between the opposing sides of the bone anchor.
 8. The bonefixation subassembly of claim 2, wherein the longitudinal axis extendsalong a longitudinal direction, the central axis extends along an axialdirection, and the select plane is defined in the longitudinal and axialdirections.
 9. The bone fixation subassembly of claim 8, wherein theguide walls define respective inner guide surfaces that extend along thelongitudinal and axial directions.
 10. The bone fixation subassembly ofclaim 2, comprising a base that defines the lower end of the anchorseat, and a pair of arms extend from the base so as to define theopposing fixation rod-receiving gaps therebetween.
 11. The bone fixationsubassembly of claim 2, comprising a collet including a collet bodyconfigured to be disposed in the bore of the anchor seat, the colletbody configured to attach to the head of the bone anchor.
 12. The bonefixation subassembly of claim 11, wherein the collet body defines anupper end and an expandable low er end, the expandable lower endconfigured to receive a head of the bone anchor.
 13. The bone fixationsubassembly of claim 12, wherein the collet defines a plurality of slotsthat extend through the expandable lower end of the collet such that theexpandable lower end of the collet defines a plurality of fingers. 14.The bone fixation subassembly of claim 13, wherein the plurality offingers includes first plurality of fingers and a second plurality offingers opposite the first plurality of fingers.
 15. The bone fixationsubassembly of claim 11, wherein the anchor seat and collet areconfigured such that, when the collet is disposed in the anchor seat,the collet is rotationally and pivotally fixed relative to the anchorseat.
 16. The bone fixation subassembly of claim 2, wherein the anchorseat body comprises a pair of spaced opposing support walls and a pairof spaced opposing spacer walls connected between the support walls. 17.The bone fixation subassembly of claim 16, wherein the anchor seat bodycomprises a pair of arms that extend up from the support walls, suchthat the spacer walls are disposed between the arms.
 18. The bonefixation subassembly of claim 16, wherein each of the spacer wallsdefines an upper end and an opposing lower end, and each upper end isshaped to conform generally with a surface of a fixation rod.
 19. Thebone fixation subassembly of claim 16, wherein the support walls flareinward toward the central axis so as to define the opposed guide walls.